This invention relates to an igniter composition for use in gas generators for safety means in motor vehicles, in particular for a vehicle occupant restraint system.
Gas generators for vehicle occupant restraint systems usually include a solid propellant on the basis of sodium azide. In addition, gas generator propellants are known which consist of a combustible, mostly nitrogen-containing organic compound as well as suitable inorganic oxidizing agents. For igniting these gas generator propellants, mixtures on the basis of boron and potassium nitrate are typically used.
The EP-A2-0736511 describes an igniter composition for azide-free gas generator propellants, which contains 5 to 100 wt-% Mg, TiH2, Al or Ti as fuels as well as 0 to 95 wt-% of a carbohydrate fuel with an oxygen content of 35 to 65 wt-%, and perchlorates or chlorates of sodium or potassium as oxidizing agents. The oxygen/fuel molar ratio of the igniter composition is at least 1, preferably at least 1.05.
The igniter compositions known from EP-A2-0736511 have a high combustion temperature, which during combustion leads to a very high content of gaseous components, for instance of potassium chloride, which gaseous components are hardly suited for priming the gas generator propellants. The known igniter compositions are also very sensitive to friction and can therefore not be brought to a manageable, precisely defined geometry or shape by jointly grinding them in a ball mill and subsequently compacting them on tabletting presses. Finally, the oxygen/fuel molar ratio of greater than 1, which is regarded as advantageous in the prior art, is not desirable, because at the high combustion temperatures a remarkable content of free oxygen is produced, which reacts with the nitrogen-containing gas generator propellants to form nitrogen oxides. WO-A-99/08983 describes an igniter composition on the basis of 5-aminotetrazole, strontium nitrate, aluminum, mica and boron nitride. Together with aluminum, the strontium nitrate used as oxidizer in this mixture only produces solid propellant residues. 5-Aminotetrazole preferably used as fuel is known to be very hygroscopic, whereby processing and storage of the known igniter compositions becomes difficult. As compared to the nitrates of guanidine compounds, such as nitroguanidine, the oxygen balance of 5-aminotetrazole is also much worse, i.e. a much higher content of oxidizer in the igniter composition is required. The igniter compositions used in WO-A-99/08983 furthermore have a positive oxygen balance which leads to the above described problems of nitrogen oxides being formed during combustion.
On the other hand, the conventionally used igniter compositions on the basis of boron and potassium nitrate mostly have a considerable oxygen underbalance, which results in large amounts of incompletely oxidized reaction products being released. These can attribute to an increase of the combustion temperature of the propellant as a result of a reaction of the incompletely oxidized reaction products of the igniter composition with constituents of the gas generator propellant.
Furthermore, B/KNO3 igniter compositions are hygroscopic and exhibit an undesired aging behavior, which may be due to the formation of boric oxides. Finally, these igniter compositions are very expensive due to the high raw material price of boron. Moreover, the same is also true for the igniter compositions described in EP-A2-0763511.
Therefore, there is still a need for igniter compositions for gas generator propellants with suitable combinations of properties, such as a good storage stability, a low hygroscopicity, moderate combustion temperatures and good processing properties.
To satisfy this need, the invention suggests an igniter composition for use in a gas generator for a safety means in motor vehicles, which igniter composition consists of a gas-generating composition in an amount of 15 to 70 wt-% and of a thermite composition in an amount of 30 to 85 wt-%. The gas-generating composition consists essentially of a guanidine compound as fuel and an inorganic oxidizer. The thermite composition consisting essentially of a metal and a metal oxide. The igniter composition or the gas-generating composition and the thermite composition, each for itself, have an oxygen balance of between 0 and xe2x88x9220%.
In accordance with the invention, oxygen balance is understood to be that amount of oxygen in percent by weight which is released upon complete reaction of the fuel to form CO2, H2O, Al2O3, B2O3 or other metal oxides (overbalancing of O2). When the oxygen present is not enough, the shortage necessary for a complete reaction is indicated with a negative sign (underbalancing of O2).
In the igniter composition in accordance with the invention, the guanidine compound is preferably selected from the group consisting of cyanoguanidine, guanidine nitrate, aminoguanidine nitrate, diaminoguanidine nitrate, triaminoguanidine nitrate, aminonitroguanidine and nitroguanidine as well as mixtures thereof. The inorganic oxidizer preferably is an alkali perchlorate or an alkali perchlorate in a mixture with ammonium perchlorate.
In the thermite composition, the metal is preferably selected from the group consisting of Al, Mg, Ti, Zr, Hf and Si or mixtures thereof. As metal oxide, there is preferably used an oxide of Si, Fe, Mn, V, Mo, Cu, Zn, Cr, Ti alone or in combination with each other.
Preferably, the thermite composition consists of Al and CuO.
The oxygen balances of the igniter composition or of the gas-generating composition and the thermite composition, respectively, vary between xe2x88x922% and xe2x88x9215%, particularly preferably between xe2x88x924% and xe2x88x9210%.
The igniter compositions according to the invention may also comprise up to 5 wt.-% usual processing aids such as trickling aids, pressing adjuvants and lubricants.
In the igniter compositions according to the invention, the gas-generating composition acts as atomizer and the thermite composition acts as supplier of particles. Together, they thus ensure a good priming of the gas generator propellant. In conventional known igniter compositions on the basis of B/KNO3, excess boron competes with the fuel of the gas generator propellant and generates additional energy during combustion to form B2O3. Moreover, this can lead to the undesired formation of carbon monoxide in the gas mixture released. These effects cannot occur in the propellants according to the invention due to the more favorable oxygen balance.
In addition, the igniter compositions according to the invention are inexpensive to produce, exhibit a much lower hygroscopicity and a high aging resistance. Moreover, priming experiments in the gas generator have revealed that with a smaller amount of the igniter compositions according to the invention an improved or at least comparable priming is achieved as compared to mixtures on the basis of B/KNO3.
Finally, it was found with the igniter compositions according to the invention that the non-gaseous reaction products advantageously are not only present as solid slag at the combustion temperature, but in a certain amount also in liquid form. The introduction of heat into the propellant to be primed is thereby improved. It is particularly preferred that at the combustion temperature of the mixture about one half of the non-gaseous reaction products of the igniter composition is present as solids and the other half is present in liquid form.
The invention will subsequently be described with reference to a few preferred embodiments. These examples should, however, merely illustrate the invention, but should not be understood in a limiting sense.